1. Field of the Invention
The present invention relates to the field of cryogenic pumping and in particular to a method and apparatus for improving the duration of operation of a cryogenic pumping system.
2. Description of the Prior Art
Cryogenic fluid, such as liquid nitrogen, is used in many industrial applications of which one includes oil well servicing. Liquid nitrogen is delivered to a well site storage tank, pumped from the storage tank, vaporized, and injected under pressure into the well bore to facilitate various drilling and oil recovery operations. One such apparatus for conversion of liquid nitrogen to gaseous nitrogen for high pressure delivery at a well head is shown and described in Zwick et al, "Fluid Pumping and Heating System," U.S. Pat. No. 4,197,712.
Typically in cryogenic delivery systems, the cryogenic fluid is drawn from a cryogenic storage tank by means of a triplex reciprocating pump. In order to effectively operate, the pump must have a net positive suction head at its inlet. In other words, given the vapor pressure of the cryogenic fluid at inlet of the pump, the net positive pressure on the cryogenic fluid must be large enough to prevent cavitation of the cyrogenic fluid during the intake stroke or cycle of the pump. Should the fluid cavitate, the pump will be ineffective to transfer the cryogenic liquid from the storage tank. In fact, such pumps require a positive input pressure which exceeds the vapor pressure of the cryogenic liquid by a predetermined magnitude. Typically 25 psi of net positive suction head is required in order for such pumps to operate or operate with any practical efficiency.
In order to partially overcome these difficulties, the prior art solution is to insert a centrifugal booster pump between the cryogenic liquid storage tank and the input of the triplex reciprocating pump. The net positive suction head required at the input of a centrifugal pump is substantially less than that required by a triplex reciprocating pumps, normally being of the order of 5 psi. The output of the centrifugal pump is then adjusted to a level to insure continued operation of the main reciprocating pump, namely typically within the range of 50 to 60 psi. Thus by supplying the lower net positive suction head at the input of the centrifugal pump, continued operation of the main triplex reciprocating pump can be insured for a longer period of time.
However, even with the centrifugal booster pump which serves as a priming pump for the main triplex pump, operation of such a cryogenic pumping circuit is limited and will eventually cease to operate. Cryogenic pumping is eventually stopped by rise in the vapor pressure of the cryogenic fluid which is delivered to the input of the triplex pump. This situation arises as follows. Generally, the pumping capacity of the main reciprocating pump is of the order of approximately 15 gallons per minute of liquid cryogenic fluid. However, at a 50 to 60 psi total pressure head applied to the input of the reciprocating pump from the output of the centrifugal booster pump, much more than 15 gallons per minute of cryogenic fluid can be supplied to the inlet of the triplex pump. If cryogenic fluid is left in a standing or near standing state, it tends to pick up heat from the ambient environment through the coils, tubing and pumps of the equipment and thus flash into a gaseous state. When this occurs further pumping of the liquid cryogenic fluid becomes difficult or impossible. Therefore, it is necessary to maintain the flow of the cryogenic fluid even when it exceeds the user's need or the current throughput volume through the triplex pump. As a result, a return line is provided from the triplex pump back to the cryogenic storage tank. A portion of the cryogenic fluid is thus recirculated between the storage tank and triplex pump. As this fluid is recirculated, it will absorb heat from the ambient environment. Slowly the temperature within the cryogenic liquid will increase due to unavoidable energy absorption during recirculation, and the vapor pressure of the cryogenic liquid will rise. Ultimately the vapor pressure will rise within the cryogenic storage tank to the point where even the minimum net positive suction head at the input of the centrifugal booster pump cannot be supplied. At this point the input to the centrifugal pump will begin to cavitate and pumping of liquid cryogenic fluid will cease. As a result the duration of operation of cryogenic pumping systems is limited.
In the application of oil well servicing, typically operation times are of the order of approximately two hours. Somewhat less commonly, oil well servicing systems will operate at their minimum capacity which extends their operational period to approximately 40 hours. Therefore prior art systems have been built with cryogenic storage tanks capable only of holding only that amount of cryogenic liquid which can be pumped during the generally expected operating times of the cryogenic system before cavitation occurs at the input to the booster pump. As stated, this has generally been a duration of approximately 2 to 40 hours. When pumping ceases, the cryogenic liquid has been exhausted from the storage tank or nearly exhausted. The remaining pressure in the storage tank is then dumped and if operations are to continue, the cryogenic storage tank must be repressurized to vent the vapor pressure, and allow boil off from the liquid to re-establish the initial or appropriate temperature of any remaining cyrogenic liquid. This necessarily entails a material loss of cryogenic liquid and a substantial downtime before pumping can resume.
What is needed then is some method and apparatus whereby the pumping duration of cryogenic systems can be extended without being subject to the shortcomings of the prior art as described above.